Add script to build distributable propane script

Rakefile

@@ -1,5 +1,9 @@
 require "rspec/core/rake_task"
 
+task :build_dist do
+  sh "ruby rb/build_dist.rb"
+end
+
 RSpec::Core::RakeTask.new(:spec, :example_pattern) do |task, args|
   if args.example_pattern
     task.rspec_opts = %W[-e "#{args.example_pattern}" -f documentation]